In coordinate-measuring machines of the portal type, drive mechanism for displacing the portal along a rectilineal guide is usually operative at the base of one of the two columns of the portal. In such machines, a problem often arises, in that, due to a change of drive speed, be it an acceleration or a deceleration, the portal is transiently displaced obliquely, or it oscillates about the vertical, due to dynamically acting inertial forces. Although the amplitude of these movements is relatively small, erroneous measurements can occur because the position of a probe element carried by the portal is then not accurately determined, since the position of the portal is, as a rule, determined by transmitters in the driven column of the portal, and these transmitters scan a scale along the guide.
In order to solve this problem, it has already been proposed in German Auslegeschrift 2,248,194 (corresponding to UK Patent 1,364,768) to arrange two parallel longitudinal scales, along opposite sides of the portal of a machine in which a carriage is transversely guided on the portal. By means of the two spaced measurement values thus obtained, the extent of oscillatory motion of the portal can be noted, and the exact position of the scanning element can be calculated, with due consideration of the instantaneous position of the transversely guided carriage.
This known solution has several disadvantages. Since the respective positions of the two portal columns are determined simultaneously as absolute values, two equivalent, highly accurate measurement systems are necessary on the two sides of the portal; and each of these systems must be connected to a stable reference point. If incremental-length measurement systems are used, it is further necessary to provide reference marks for the aforementioned reference point on both scales, so that the measurement systems can always be reinitialized.
Furthermore, temperature gradients can occur within the machine, and the influence of temperature on the two scales is not the same. Thus, as compared with measurement via a single scale, twice the number of temperature sensors are needed to read the scales, and mathematical correction of the influence of the temperature becomes very complicated.